(1) Field of the Invention
The present invention relates to an image processing apparatus for processing image data such as half tone and line images (e.g., characters), or code data such as a compression code.
(2) Description of the Prior Art
Ink jet, thermal transfer and laser beam printers are conventionally known as image processing apparatuses for reproducing a half tone image from a dot image. In order to reproduce the half tone image in these apparatuses, a dither method or a density pattern method is used to reproduce the half tone image by dot modulation of a small region. In particular, the dither method is mainly used to reproduce the half tone image by a color laser beam printer.
Image data supplied from a color video camera or an image file to the color printer of the type described above is temporarily stored in a buffer memory in the color printer and is read out therefrom. In this case, the color printer performs dither processing to print out a dot image obtained in accordance with the density of the transferred image. However, when only one type of image buffer memory is used, character data is mixed with the half tone image data. As a result, the edge sharpness of the character image is degraded by dither processing, resulting in inconvenience.
In particular, in an image consisting of characters and lines with high contrast, the character and line edges become vague after dither processing. In addition to this disadvantage, the density of the solid image portion is decreased, resulting in a loss of image sharpness.
In addition, when a half tone image is superimposed onto characters, for example, the half tone image is copied by a known copying apparatus. The resultant copying sheet is used as a printing sheet and is printed with the characters by a hard copy printer. However, the operation of this method is time-consuming and cumbersome. Furthermore, it is difficult to align the image with the characters, resulting in an impractical application.
On the other hand, a composite type electrophotographic copying apparatus is known, as shown in FIG. 1. This apparatus has a copying unit and a line printer (e.g., laser beam printer). A copy image of an original placed on an original table 150 is superimposed on a printer character output supplied from an external device (not shown) such as a host computer through a signal line 163. The composite image is then printed out on a copying sheet 161. The original table 150 is illuminated by an exposure lamp 151, and light reflected thereby is focused on the surface of a photosensitive drum 159 through mirrors 152 to 155 and a lens 156, so that a latent image corresponding to the original image is formed on a surface portion of the photosensitive drum 159. On the other hand, character data is supplied from the external device (not shown) to an interface control circuit 162 where it is converted to dot data. The dot data modulates a laser beam generated from a semiconductor laser 164. A polygonal mirror 157 serves to horizontally scan the laser beam. When the laser beam is modulated and scanned in this manner, a latent character image is superimposed on a latent original image on the surface of the photosensitive drum 159. Thereafter, the normal electrophotographic process of consecutive development, transfer, and fixing is performed on a composite image consisting of both the half tone and character images. According to the apparatus described above, the half tone image is formed independently of the character image and is printed out. For example, when characters are superimposed on a substantially dark portion of the half tone image, the characters cannot be easily read, as shown in FIG. 2. In particular, the characters superimposed on the solid portion cannot be read entirely.